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Tropical Pacific Conditions:
“La Nada” Neutral phase?
Climate Prediction Center
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Important Winter Terms
El Niño–Southern Oscillation (ENSO) is frequently the main influence on weather in the United States and around the world, especially in late fall, winter and early spring.
El Niño (Warm Phase)
When the Walker circulation (conceptual model of the air flow gradient in the tropics in the lower atmosphere or troposphere/High Pressure Eastern Pacific/Low Pressure Indonesia) weakens or reverses and the Hadley circulation (tropical atmospheric circulation that features air rising near the equator mainly caused by solar heating) strengthens an El Niño results. The conditions cause the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all offshore northwestern South America. El Niño is associated with a band of warmer than average ocean water temperatures that periodically develops off the Pacific coast of South America. ‘El Niño–Southern Oscillation’ (ENSO) refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean and in air surface pressure in the tropical western Pacific. The El Niño warm oceanic phase accompanies high air surface pressure in the western Pacific. Mechanisms that cause the oscillation are a developing area of research.
El niño is Spanish for “the boy”, and the capitalized term El Niño refers to the Christ child, Jesus, because periodic warming in the Pacific near South America is usually noticed around Christmas.
La Niña Cold phase
An especially strong Walker circulation causes a La Niña, resulting in cooler ocean temperatures in the central and eastern tropical Pacific Ocean. The condition is caused by increased upwelling — an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, which replaced the warmer, usually nutrient-depleted surface water.
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño Southern Oscillation climate pattern. During La Niña, the sea surface temperature across the equatorial eastern central Pacific will be lower than normal by 3–5 °C.
In the United States, an appearance of La Niña happens for at least five months of La Niña conditions. However, each country and island nation has a different threshold for what constitutes a La Niña event, which is related to each country’s specific interests.
The name La Niña originates from Spanish, meaning “the girl”, analogous to El Niño meaning “the boy”.
“La Nada” Neutral phase
If the temperature variation from climatology is within 0.5 °C (0.9 °F), ENSO conditions are described as neutral. Neutral conditions are the transition between warm and cold phases of ENSO. Ocean temperatures (by definition), tropical precipitation, and wind patterns are near average conditions during this phase. Close to half of all years are within neutral periods. During the neutral ENSO phase, other climate anomalies/patterns such as the sign of the North Atlantic Oscillation or the Pacific–North American teleconnection pattern exert more influence.
The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles. The Polar Vortex ALWAYS exists near the poles, but weakens in summer and strengthens in winter. The term “vortex” refers to the counter-clockwise flow of air that helps keep the colder air near the Poles.
Frequently during winter in the northern hemisphere, the polar vortex will expand, pushing cold air southward with the jet stream. This occurs fairly regularly during wintertime and is often associated with large outbreaks of Arctic air in the United States. The Polar Vortex that occurred January 2014 is similar to many other cold outbreaks that have occurred in the past, including colder outbreaks in 1977, 1982, 1985 and 1989. [More on Polar Vortex from NWS …]
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NWS Winter Terms
Blizzard Warnings are issued for frequent gusts greater than or equal to 35 mph accompanied by falling and/or blowing snow, frequently reducing visibility to less than 1/4 mile for three hours or more. A Blizzard Warning means severe winter weather conditions are expected or occurring. Falling and blowing snow with strong winds and poor visibilities are likely, leading to whiteout conditions making travel extremely difficult. Do not travel. If you must travel, have a winter survival kit with you. If you get stranded, stay with your vehicle and wait for help to arrive.
Winter Storm Warnings are issued for a significant winter weather event including snow, ice, sleet or blowing snow or a combination of these hazards. Travel will become difficult or impossible in some situations. Delay your travel plans until conditions improve.
Ice Storm Warnings are usually issued for ice accumulation of around 1/4 inch or more. This amount of ice accumulation will make travel dangerous or impossible and likely lead to snapped power lines and falling tree branches. Travel is strongly discouraged.
Wind Chill Warnings are issued for a combination of very cold air and strong winds that will create dangerously low wind chill values. This level of wind chill will result in frostbite and lead to hypothermia if precautions are not taken. Avoid going outdoors and wear warm protective clothing if you must venture outside. See the NWS Wind Chill Chart.
Lake Effect Snow Warnings are issued when widespread or localized lake induced snow squalls or heavy showers are expected to produce significant snowfall accumulation. Lake effect snow usually develops in narrow bands and impacts a limited area. These bands can produce very heavy snow with sudden restrictions in visibility. Driving conditions may become hazardous at times.
Weather Warnings — National Weather Service Warnings, Watches and Advisories and Criteria